The contrast in projection displays is limited by the amount of ambient light impinging on the viewing surface. This problem is acute in a front projection display which projects an image onto a conventional diffusing surface. The viewing screen must be highly reflective due to the relatively low light levels generated by common projection technology. In addition, current projection techniques generate non-saturated colors requiring a surface that can reflect across a broad spectral band. As a result, these screens reflect the ambient light equally as well as the projected light from the display. The darkest screen color is white when any appreciable room light is present.
In order to generate high contrast images, the projector must generate sufficiently high light levels so that the viewer can perceive the projected image above the ambient light levels. Most projectors cannot currently meet this requirement. Generating high contrast, sufficiently bright images using front projection techniques has not yet been adequately solved.
The most common solution is to turn the room lights off. However, this solution is usually not acceptable in, for example, educational and training meetings. Several methods have been developed to solve this contrast problem. One method employs a rear projection screen. Several designs exist which enable the viewing screen to function as an optical diode. In this case, the projected image is transmitted through the viewing surface while the ambient light is absorbed by the screen or transmitted through the screen. These devices greatly improve the contrast and remain the preferred embodiment for projection displays.
This optical diode effect is the basis of the standard screens used in rear projection televisions. These screens employ a lenticular lens element located directly behind a mask of horizontal black stripes. The absorbing regions are separated only by a small gap. The lenticular lens focuses the projected image generated from behind the screen through the gaps towards the viewer. However, the ambient light impinging from the front is absorbed by the black stripes. The small fraction of ambient light transmitted through the gaps is absorbed by the blackened walls of the projector""s cabinet.
The common optical diode screens have several limitations. The obtainable contrast is still below the level that is required, particularly in high ambient light conditions as in an office environment. In addition, most diodes employ fixed structures and optics as discussed above. These surfaces lead to moire patterns and limit the obtainable resolution. No spatial frequencies can be displayed which have a higher frequency than the periodic pattern of the structure.
One approach to improve contrast and resolution is to take advantage of the light emission characteristics offered by the new projection techniques based on lasers or resonant microcavities. The latter is described in U.S. Pat. No. 5,469,018. These light sources generate narrow band emission, resulting in highly saturated primary colors. In contrast, common light valves and projection CRTs generally produce non-monochromatic light. Any spectral filtering employed to increase saturation limits the brightness and lowers the contrast.
Viewing screens can be fabricated to generate high contrast images when the image sources emit monochromatic light. In a front viewing screen application, such surfaces can be made to preferentially reflect the projected light while absorbing the ambient light. In a rear viewing screen application, such surfaces can be made to transmit the projected image while absorbing the ambient light.
Accordingly, the invention includes a viewing surface capable of high contrast and high resolution comprising one or more materials that preferentially reflect or transmit or scatter monochromatic light at the primary wavelengths necessary to generate the color gamut appropriate for a given display application.
Further, the invention includes a viewing surface capable of high contrast and high resolution comprising one or more materials which can include rare earth ions, etc. and may be combined with absorbing substrates and/or interference filters.